The further development of a general and flexible methodology for the elaboration of polypropionate subunits, which are a common feature of macrolides and ionophores will be addressed. This will be done by means of oxirane chemistry coupled to other stereoselective synthetic transformations. Carbonate extension and iodolactonization reactions are key to this procedure. Initially a general hydroxyl group inversion methodology will be studied in detail in order to complement the all syn- polyol sequence generated from the carbonate extension reaction. This will allow for the construction of a polypropionate unit in which the hydroxyl functions can have any given stereochemistry. Alternatively an epoxide inversion sequence will also be developed. This will be based on the regioselectivity of the hydroxyl assisted oxirane opening and will allow for the simultaneous inversion of two centers. A variation of the carbonate extension reaction in which the intermediate iodocarbonate is transformed into an iodoacetonide will be studied. This new intermediate is a masked hindered oxirane and its electrophilic chemistry will be studied. This can be a solution to the lack of reactivity of hindered epoxides. Finally, we will like to apply this methodology to the synthesis of the polyketide chain of the ansamycin antibiotics rifamycin S and streptovaricin V. These compounds have received much attention due to their notable biological activity. This will be accomplished via the general reiterative procedure which we have been developing together with the above studies. Although this methodology is intended for these targets, it can be extended to the synthesis of other families of products. In summary, it is intended to develop and improve methodologies for the synthesis of these important biologically active targets as a contribution to this expanding field.